In an image forming apparatus such as a laser printer, the toner is generally contained in a developer cartridge. The toner is consumed when image forming is repeated, thereby decreasing the remaining amount of toner in the developer cartridge.
In Japanese Unexamined Patent Application Publication No. 2004-301896, an apparatus is provided for detecting the remaining amount of toner in a developer cartridge. A toner container that is detachable from an image forming apparatus is disclosed. A second light guide is installed on the top of the toner container while a first light guide is installed on the bottom. Light emitted from an LED travels inside the toner container through the first light guide, and then travels through the second light guide to be received by the phototransistor. The remaining amount of toner in the toner container is estimated, based on the result of analysis of the received light. The LED and phototransistor are located on the same board. The light from the LED is deflected by about 90° by the first light guide to travel through the toner container from the bottom to the top. The traveling light is deflected by about 90° by the second light guide. The light traveling through the second light guide is received by the phototransistor.
When the toner container is detached from the image forming apparatus, the light from the LED is not received by the phototransistor, because the light cannot be deflected. On the other hand, if a new toner container is attached to the image forming apparatus, the light from the LED is blocked by the toner from traveling further, so the light does not reach the phototransistor, because the amount of toner in the container is full.
Therefore, in the detector described in Japanese Unexamined Patent Application Publication No. 2004-301896, the state in which the toner container is detached from the image forming apparatus cannot be distinguished from the state in which a new toner container is attached to the image forming apparatus.